The present invention relates generally to optical communications and, more particularly, to optical phase conjugation aided long-haul transmission system with enhanced signal-to-noise ratio and nonlinear tolerance.
As 100-Gb/s coherent optical communication systems are commercially available for long-haul optical transmission systems. Linear distortion such as chromatic dispersion (CD) and polarization mode dispersion (PMD) can be completely compensated by digital signal processing (DSP) algorithms in principle. Thus optical noise accumulation and non-linear impairment are the main factors that limit system reach. One way to overcome optical noise accumulation is to increase signal power. However, in fiber links higher signal power induces more non-linear impairment as well, which harms the signal quality and shorten the system reach.
In previous works, degenerated and non-degenerated phase sensitive amplifiers (PSAs) are proposed to increase the optical signal-to-noise ratio (OSNR) and mitigate the nonlinear phase noise. However, due to the high requirement of the phase match between the pump and signal, both approaches need high quality carrier regeneration technology and feedback optical phase lock loop (PLL) to maintain phase coherence between the pump and signal. Moreover, they require twice of phase conjugation operations done in optical domain, which requires two sets of optical phase conjugation setups. Therefore, the system setup is much more complicated and the system cost is largely increased.
Accordingly, there is a need for a a long haul transmission system that overcomes limitations of prior efforts.